The Cell Structure and Function
The cell is the lowest level of structure capable
of performing all the activities of life.
The first cells were observed and named by
Robert Hooke in 1665 from slice of cork.
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The Cell Thoery
Proposed by Matthais Schleiden and Theodor
Schwann in 1839:
-All living things are made up of cells.
-Cells are the smallest working unit of all
living things.
-All cells come from preexisting cells through
cells division.
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Some organisms consist of a single cells =
unicellular organism, others are multicellular
aggregates of specialized cells.
3
Whether multicellular or unicellular, all
organisms must accomplish the same functions:
uptake and processing of nutrients
excretion of wastes
response to environmental stimuli
and reproduction among others.
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How We Study Cells?
-microscope
-cell fractionation
Most cells are between
1-100 µm in diameter which
can be visualized by light
microscope.
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Light microscope (LM)
In the LM, visible light is passed through the
specimen and then through glass lenses.
The lenses refract light such that the image is
magnified into the eyes or the video screen.
magnification and resolving power
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Magnification = the ratio of an object’s image to its
real size.
Magnification of LM ~ X1,000
Resolving power = the measure of the image clarity.
It is the minimum distance two points can be
separated and still viewed as two separate points.
Resolution is limited by the shortest wavelength
of the light ~ one half of the wavelength used.
The minimum resolution of a light microscope
is about 2 microns, the size of a small bacterium.
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Electron microscope (EM)
-focuses a beam of electrons through the specimen
or onto its surface
-theoretically, the resolution of a modern EM could
reach 0.1 nanometer (nm), but the practical limit is
closer to about 2 nm
-Transmission and Scanning electron microscope
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Transmission electron microscope (TEM)
-study internal ultrastructure of cells
-electron beam was aimed through the thin section
of specimen
-the image was focused and magnified by
electromagnet (instead of glass lenses)
-to enhance the image,
the thin section of
preserved cells are
stained with atoms of
heavy metals
-the micrograph is 2
dimensional
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Scanning electron microscope (SEM)
-study the surface structure of the cells
-the sample surface is covered with the thin film of
gold
-the electron beam excites the electrons on the
sample surface
-the secondary electrons
are collected and focused
on a screen
-the image appeared 3dimensional
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Cell Fractionation
-to separate the organelles of cells for functional
study
-the disrupted cells are centrifuged at different
speed and duration to fractionate components of
different sizes
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Cell fractionation prepares quantities of
specific cell components/organelles for functional
analysis = biochemical studies.
Microscopes are a major tool in cytology = the
study of cell structures.
Cytology coupled with biochemistry, the study
of molecules and chemical processes in metabolism,
developed modern cell biology.
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A Panoramic View of the Cell
Basic features of cells:
-All cells are bounded by a plasma membrane.
-The semifluid substance within the membrane is
the cytosol, containing the organelles.
-All cells contain chromosomes which carry genes in
the form of DNA.
-All cells also have ribosomes, tiny organelles that
make proteins using the instructions contained in
genes.
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-A major difference between prokaryotic and
eukaryotic cells is the location of chromosomes.
-eukaryotic cell, chromosomes are contained in
a membrane-enclosed organelle, the nucleus.
-prokaryotic cell, the DNA is concentrated in
the nucleoid without a membrane separating it from
the rest of the cell.
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Bacteria: a prokaryotic cell
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Problems with Cell Size
In general, eukaryotic cells are much bigger than prokaryotic
cells:
Most bacteria are 1-10 microns in diameter.
Eukaryotic cells are typically 10-100 microns in
diameter.
-A large cell requires "much more" in terms of the
cellular components.
-Uptake from the environment is also a problem for
large cells: there is less surface area compared to the
volume.
-Distribution of nutrients from one portion of a large
cell to another is also a problem, simply because of the
distance required for the nutrients to travel.
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Thus most cells are small: sufficient surface area to
accommodate the volume.
Larger organisms do not generally have larger cells than smaller
organisms, - just simply more cells.
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The Plasma Membrane
-double layer of phospholipids
-various proteins are attached to it
-carbohydrate side chains are found only on the outer surface
of plasma membrane
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The Plasma Membrane
- function of plasma membrane = selective barrier that
allows passage of oxygen, nutrients, and wastes for the
whole volume of the cell.
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Overview of Animal Cell
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Overview of Plant Cell
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The Nucleus and Its
Envelope
-The nucleus
contains most of the
genes in a eukaryotic
cell.
Some genes
are located in
mitochondria and
chloroplast
-The nucleus
averages about 5
microns in diameter.
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-The nucleus is
enclosed by a
nuclear envelope
which is a double
membrane of 20 -40
nm apart.
-Where the double
membranes are
fused, a nuclear
pore complex allows
large
macromolecules and
particles to pass
through.
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-The nuclear side of
the envelope is lined
by the nuclear
lamina, a network of
intermediate
filaments that
maintain the shape
of the nucleus.
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Level of Chromatin Packing
In the nucleus,
the DNA and the
associated proteins are
organised into firous
material called
chromatin.
In a normal cell
they appear as diffuse
mass.
26
However, when the
cell prepares to divide, the
chromatin fibers coil up to
be seen as separate
structures, chromosomes.
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Nucleolus = a region (in
the nucleus) of densely
stained fibers and
granules adjoining
chromatin.
In the nucleolus,
rRNA is synthesized and
assembled with proteins
from the cytoplasm to
form ribosomal subunits.
The subunits pass
from the nuclear pores to
the cytoplasm where
they combine to form
ribosomes.
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Ribosomes
-particles consisted of proteins and ribosomal RNA
(rRNA)
-function = protein synthesis
-prokaryotes = 70S eukaryotes = 80S
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Eukaryotes (80s ribosome):
large subunit = 60S
45 proteins
rRNA = 28S, 8S and 5S
small subunit = 40S
30 proteins
rRNA = 18S
30
-free ribosomes in cytosol synthesize proteins that
function within cytosol
-bound ribosomes are attached to the outside of the
endoplasmic reticulum or nuclear envelope:
membrane protein, organelle proteins or secretory
protein
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Bound and free ribosomes are structurally
identical and can alternate between the two roles.
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The Endomembrane System
The collection of membranes inside and around
a eukaryotic cell.
These membranes are related either through
direct physical contact or by transfer of vesicles (sac
of membrane).
In spite of these links, these membranes have
diverse functions and structures:
nuclear envelope, endoplasmic reticulum,
Golgi apparatus, lysosomes,
vacuoles, and the plasma membrane.
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Endoplasmic reticulum (ER)
-ER consists of a network of
membranous tubules and sacs
called cisternae. (cisterna = a
reservoir for a liquid)
-the network are interconnected
- The ER membrane is
continuous with the nuclear
envelope and the cisternal space
of the ER is continuous with the
space between the two
membranes of the nuclear
envelope.
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-smooth and rough ER (with &
without ribosomes)
-Smooth ER:
synthesis of lipid (oils,
phospholipids, and steroids)
glycogen metabolism in
the liver cells
detoxification of drugs
and poisons
store calcium for muscle
contraction
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Rough ER: ribosomes are
attached to the outside
-is abundant in cells that
secrete protein
-synthesis secretory
proteins, cell membrane protein
and organelle protein
(proteins are targeted to determined
location according to the sorting signals.
Sorting signals are encoded in a.a
sequences or in the attached
oligosaccharides)
-synthesis of
phospholipids and ER
associated protein
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-proteins are synthesized from
the bound ribosomes
-threaded into the cisternal
space through a pore formed
by a protein in the ER
membrane
-protein folds into its native
conformation
-an oligosaccharide is attached
to the protein = glycosylation
-those proteins are wrapped in
the transport vesicles that bud
from the ER
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The Golgi apparatus:
-major sites for carbohydrate symthesis
-sorting and dispatching station for the products of the ER
-consists of flatten membranous sacs, cisternae
-unlike ER cisternae, the Golgi cisternae are not physically
connected.
38
-a Golgi stacks has polarity: cis face (the receiving
side) and trans face (the shipping side)
39
-transport vesicle from ER fuses to the cis face to transfer the
material to the Golgi
-proteins and lipids are altered; glycosylation and
phosphorylation (tagging the sorting signal)
-oligosaccharides portion of the glycoproteins are modified:
Golgi removes some sugar monomers and substitutes others
-some polysaccharides are synthesized in the Golgi e.g pectin
and cellulose of the plant cell wall and most of the
glycosaminoglycans of animal extracellular matrix
-Golgi products (that will be secreted) depart from the trans
face of the Golgi by transport vesicle for the correct docking.
40
Relationships among Organelles of the
Endomembrane System
-secretory proteins,
lysosomes, vacuoles
and membrane are
synthesized by the
rough ER, then
transported through
the Golgi as a vesicle.
-during this process, their molecular compositions
and metabolic functions are modified.
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Lysosomes: principal sites of intracellular digestion
-contain hydrolytic enzymes (required acidic pH) to digest
proteins, polysaccharides, fats and nucleic acids
(if those hydrolases leak out of the lysosmes, they are not likely to do
damage unless the cells become acidic)
42
-lysosomal enzymes and membranes are made by rough
ER and transferred to the Golgi for processing
-lysosomal membranes are highly glycosylated to protect
them from lysosomal proteases
43
-Food particles engulfed as a food vacuole (phagocytosis)
or an endosome (product of receptor-mediated
endocytosis) is fused with the lysosome.
-The digestion products are passed to cytosol and
become nutrient for the cell.
44
-autophagy = process which cells recycle its own
organic material
-the organelles are fused with a lysosome
-after digestion, the organic monomers are
returned to the cytosol for reuse
45
Vacuoles:
-membrane-bound sacs
-diverse functions in cell maintenance
food vacuoles formed by phagocytosis and
digested by lysosomes
contractile vacuoles (in protists) pump excess
water out of the cells
central vacuole (a versatile compartment in
plants) stores protein and metabolic by-products,
reservoir of inorganic ions, pigments
46
47
Mitochondria and Chloroplast
-both are energy transformers of cells
mitochondria = cellular respiration
chloroplast = photosynthesis
-both are not part of the endomembrane system
-most of their proteins are synthesized by the free
ribosomes in the cytosol
-a few of the proteins are synthesized from their own
ribosomes
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-both organelles contain small quantity of DNA that
direct the synthesis of polypeptides produced by the
internal ribosomes
-both organelles grow and reproduce as
semiautonomous organelles
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Mitochondria
-1-10 µm long
-some cells contain a single large mitochondrion but
most cells contain several mitochondria
-enclosed by two membrane: outer and inner
membrane with different permeability
-cristae = fold innermembrane to increase the
surface area
-matrix and intermembrane space
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-matrix contains:
-ds circular DNA
-prokaryote like ribosome (70S)
-enzymes in TCA cycle
-enzymes for β-oxidation of fatty acid
Glucose and fatty acids are catabolized in the
matrix of mitochondria.
-innermembrane of mitochondria contains:
-electron transport chain
-ATP synthase
The energy from catabolism in the matrix is
converted into ATP.
52
Chloroplast
-is one of the generalized plant structure called
plastids
-found in mesophyll cells of the leaves and in algae
-2-4 µm wide and 5-10 µm long
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-2 membranes: inner and outer membrane
-stroma ~ matrix of mitochondria
*ds circular DNA
*70S ribosomes
*enzyme for carbohydrate biosynthesis
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-thylakoids contain photosynthetic machinery of the
chloroplast:
*light absorbing pigment
*electron carriers
*ATP synthesizing apparatus
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Microbodies
-electron-dense cytoplasmic particles bound by a single
membrane
-contain oxidative enzymes: D-amino acid oxidase,
ureate oxidase and catalase
-they are not formed in the
Golgi complex
-they are self replicating, like
the mitochondria
-e.g. peroxisomes, glyoxisomes
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Peroxisomes
-single membrane
-contain enzymes that transfer
hydrogen from various
substrate to oxygen and
produce H2O2 as intermediate
product
-but peroxisomes contain
another enzyme that convert
H2O2 to H2O
-some peroxisomes break down fatty acids to smaller molecules
that are transported to mitochondria for fuel
Glyoxysomes = specialized peroxisomes (found in fat storing
tissues of plant seeds) convert fatty acid to sugar which can be
used as energy for seedling.
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The Cytoskeleton
-a network of fibers extending throughout the cytoplasm
-function:
provide mechanical strength to the cell
establish cell shape
locomotion (several types of cell motility)
intracellular transport of organelles
-3 main types of fiber:
1.microtubules: determine the positions of membraneenclosed organelles and intracellular transport
2.microfilament: determine the shape of the cell and
necessary for the whole cell locomotion
3.intermediate filament: provide mechanical strength
and resistance to shear stress
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Cytoskeleton and Cell Motility
Cell motility requires interaction of
cytoskeleton with proteins called motor molecules
in ATP dependent manner.
Sliding of
neighboring
microtubules moves
cilia and flagella.
In muscle contraction, motor molecules slide
microfimaments.
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Walking of organelles along microtubules
Motor molecule attached to receptor on
organelles can “walk” the organelles along
microtubules or microfilaments.
e.g. migration of
neurotransmitters
containing vesicles to
the tips of nerve cell
axon.
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Microtubules
-are straight, hollow
cylinders
-have a diameter of
about 25 nm
-are variable in length
but can grow 1000 times as
long as they are thick
-are built by the
assembly of dimers of alpha
tubulin and beta tubulin.
-are found in both
animal and plant cells
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-grow at each end by the
polymerization of tubulin
dimers (powered by the
hydrolysis of GTP)
-shrink at each end by
the release of tubulin dimers
(depolymerization)
-participate in a wide
variety of cell activities. Most
involve motion. The motion is
provided by protein "motors"
that use the energy of ATP to
move along the microtubule.
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Microtubule motors
There are two major groups of microtubule motors:
kinesins (most of these move toward the plus end
of the microtubules) and
dyneins (which move toward the minus end).
Examples:
The rapid transport of organelles, like vesicles and
mitochondria, along the axons of neurons takes place
along microtubules with their plus ends pointed toward
the end of the axon. The motors are kinesins.
The migration of chromosomes in mitosis and
meiosis takes place on microtubules that make up the
spindle fibers. Both kinesins and dyneins are used as
motors
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-function of microtubules:
*shape and support the cell
*resist compression
*serve as track along which organelle can
move via motor molecule
*responsible for separation of chromosome
during cell division
*component of centrosome, centrioles, cilia
and flagella
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Centrosomes and Centrioles
Centrosomes (microtubule-organizing center) = a
region near the nucleus from which microtubules
sprouts.
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Centrioles
-each centrosomes contain
a pair of centrioles
-found in animal cells
-composed of 9 sets of
triplet microtubules
arranged in a ring
-centrioles replicate before
cell division
-may help organize
microtubule assembly
but centrioles are not essential for this function:
centrosomes of most plants lack centrioles.
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Cilia and Flagella
-both cilia and flagella are constructed from
microtubules
-both provide either locomotion for the cell or move
fluid pass the cell
-found in prokaryotes and eukaryotes
-cilia and flagella differ in their beating pattern
cilia sweep
mucus carrying
trapped debris
from the lungs.
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A flagellum has an undulating motion that
generates force in the same direction as the
flagellum’s axis.
69
Cilia works like oars. The alternating power and
recovery strokes generating force in a direction
perpendicular to the cilium’s axis.
70
71
-celia and flagella have the same ultrastructure =
microtubules core sheathed in an extension of
plasma membrane
-“9+2” arrangement of microtubules (9 doublets +
2 single)
-both are anchored in the cell by a basal body,
whose structure is identical to a centriole
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-each microtubule
connects to the next by a
motor molecule dynein
-dynein is responsible for
the bending of cilia and
flagella
-addition of phosphate group from ATP to dynein
causes conformational change in this protein which
result in grabing, moving and releasing of the outer
microtubule
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Microfilament or Actin Filament
-a twisted double chain of
actin subunits of 7 nm
diameter
-bear tension (pulling force)
-found in all eukaryotic cells
-combine with other protein to form a threedimensional network beneath the plasma
membrane
74
-Bundles of microfilament
make up core of microvilli (to
increase cell surface area) of
the animal cells specialized in
transport materials across
plasma membrane.
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-this network make the semisolid consistency of the
cortex = gel state in the outer cytoplasmic layer
-the interior cytoplasm is the more fluid state
(sol)
-cytoplasm can be converted between gel-sol due
to the reversible assembly of microfilaments into
the networks
Amoeboid movement
:Amoeba, white blood
cells
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Cytoplasmic streaming: plant cells
-a circular flow of cytoplasm within cells
-this movement is a result of actin-myosin
interactions and sol-gel transformations
-speed the distribution of materials within the cells
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Muscle fiber
-made of actin filaments arranged parallel to one
another along the length of a muscle cell
-a motor molecule myosin is interdigitated with actin
filament
-muscle contraction results from the sliding of actin
and myosin
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Intermediate filament
-cytoplasmic ropelike fibers with average 10 nm in diameter
(and thus are "intermediate" in size between actin filaments
(8 nm) and microtubules (25 nm)
-There are several types of intermediate filament, each
constructed from one or more proteins characteristic of it.
keratins are found in epithelial cells and also form hair
and nails (over 20 different kinds of keratins have been found)
nuclear lamins form a meshwork that stabilizes the
inner membrane of the nuclear envelope
neurofilaments strengthen the long axons of neurons;
vimentins provide mechanical strength to muscle (and
other) cells.
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Despite their chemical diversity,
intermediate filament play similar
roles in the cells: providing a
supporting framework within the
cells
80
-other intermediate filaments make up the nuclear
lamina that lines the interior of the nuclear envelope
81
The vimentin intermediate filament network (red) extends into
the lamella where tetramers colocalize with the actin-bundling
protein, fimbrin (green). These fluorescent foci are specialized
focal adhesions found in osteoclasts and macrophages called
podosomes
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-intermediate filaments strengthen the long extension
(axon) of nerve cells that transmit impulse
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